Each loudspeaker generates natural resonance vibration frequency. Therefore, the reproduction level fluctuates extraordinarily at the resonance vibration frequency compared with other frequencies, so that it is difficult to flatten reproduction frequency characteristics. As a result, a signal can not be reproduced appropriately.
To solve this problem, Unexamined Japanese Patent Publication No. H7-162992, discloses that the resonance level of a diaphragm at the natural vibration frequency can be suppressed by making the outer periphery of the diaphragm an oval.
As in the example discussed above, making the outer periphery of the diaphragm an oval figure, for example, has a certain effect of suppressing the resonance level at the natural vibration frequency. However, the high resonance level at the natural vibration frequency remains a problem.